Striasteroids A–C, Three Hybrid Steroids with Neuraminidase Inhibitory Activities from a Marine-Derived Striaticonidium cinctum

Three unprecedented hybrid steroids, striasteroids A-C (1-3), together with a known analogue (4), were isolated from the fungus Striaticonidium cinctum SCSIO41432. Their structures were identified through spectroscopic data, X-ray crystallography, and electronic circular dichroism calculations. Compounds 1 and 2, which feature a new 6/6/6/6/6/5 hexacyclic framework, are polyketide-steroid hybrids with an oxabicyclo[3.3.1]nonane-bridged system. Compound 3, characterized as the first naturally occurring adenine-steroid hybrid, exhibited the most potent inhibitory activity against neuraminidase with an IC50 value of 6.15 ± 1.15 μM.